When a mobile phone is operable in many different frequency bands, such as GSM and W-CDMA bands, the RF front-end of the mobile phone generally has a plurality of switches for selecting among those bands. For example, a mobile phone is operable in a plurality of GSM bands and one W-CDMA band, it has two antennas: one GSM antenna 10 and one W-CDMA antenna 12, as shown in FIG. 1a. The front-end module 1 includes an antenna switch 20 having a plurality of series switches 22, 24, 26 and 28 operatively connected to the GSM signal paths 32, 34, 36, and 38 so as to allow the GSM signals to be routed through the GSM antenna 10 through a common node 29. In addition, a duplexer 40 having a bandpass filter 42 and a bandpass filter 44 operatively connected to the W-CDMA signal paths 52, 54 so as to allow the W-CDMA signals to be routed through the W-CDMA antenna 12. Each of the switches 22, 24, 26 and 28 in the antenna switch 20 serves as a path selection component that must be operated in an “ON” state or an “OFF” state for path selection purposes. The bandpass filters 42 and 44 can also be considered as path selection components, but they do not need to be operated in the “OFF” state for band rejection. Unlike a shunt switch, each of these path selection components is a series component, which is connected in series with other RF-front end components such as amplifiers, filters (not shown) along the same signal path. In practice, the multimode, multiband RF front-end is substantially like that shown in FIG. 1b. The front-end typically comprises a plurality of bandpass filters (such as filter 332), baluns (such as balun 432), matching elements (such as inductive element 836 and capacitive element 834) and phase-shifters (such as shifters 725, 734, 752). In addition, the RF front-end comprises a diplexer 920 with harmonic filters for GSM Tx. For GSM Tx-Rx switching, the actual implementation for each switching function would have at least one series switching element and one shunt element. The switching elements can be FETs such as CMOS devices, or p-HEMTs such as GaAs devices, or the like. In case of diodes being used as switches, one or more 90 degree phase shifters are also used.
As shown in FIG. 1b, For GSM Tx-Rx switching, two series diodes 526, 528, two shunt diodes 624, 626 and two phase-shifters 725 and 734 are used. If FETs are used as switching elements, as shown in FIG. 1c, any one of the paths can be selected by biasing the appropriate series switch “ON” (here the second path is selected). Only the shunt switch in the selected path is “OFF”, while all remaining shunt switches are “ON” in order to increase isolation with respect to the selected path.
The antennas 10 and 20 as shown in FIG. 1a are herein referred to as internal antennas of the mobile phone because they usually come with the mobile phone. In the example of FIG. 1a, the antenna switch 20 is used for selecting the signal paths among two different GSM bands. In many cases, the antenna switch must be used for selecting the signal paths among four different GSM bands at 850, 900, 1800 and 1900 MHz.
When the mobile phone is used in connection with an external antenna, such as when the mobile phone is so configured that it can be used to receive and transmit signals through one or more automobile antennas, additional switches are required. As shown in FIG. 2, two additional switches 62, 64, which are separately connected in series with the switches 22, 24, 26 and 28, are used for selecting the signal paths between the internal antenna 10 and the external antenna 14 through the common node 29. Likewise, two additional switches 66, 68, which are separately connected in series with the bandpass filters 42, 44, are used for selecting the signal paths between the internal antenna 12 and the external antenna 14. As with the internal switches 22, 24, 26 and 28, each of the additional switches 62, 64, 66 and 68 is also a path selection component that is operable in an “ON” state and an “OFF” state. Thus, in the switch arrangement as shown in FIG. 2, each of the signal paths 32, 34, 36 and 38 has two series path selection components, each operable in an “ON” state and an “OFF” state, regardless of which of the antennas 10 and 14 is being used for signal reception and transmission. For example, if the path between the external antenna and the GSM Rx1 path is selected, the receive signals must pass through two path selection components connected in series: switch 64 and 22. Putting an additional path selection component in the signal path introduces additional losses. The losses are especially critical in the W-CDMA case. It should be noted that the switches as shown in FIGS. 1 and 2 are typically solid-state switches such as PIN diodes, GaAs p-HEMTs (pseudomorphic High Electron Mobility Transistors), for example. The losses associated with these types of solid-state switches are generally quite significant. Although one could use mechanical switches to reduce the losses, the size of mechanical switches renders their usage impractical.
It is desirable and advantageous to provide a method and device for selecting signal paths between internal and external antennas so that the losses can be avoided or minimized.